diff --git a/BookAStar/BookAStar/Pages/EditBillingLinePage.xaml b/BookAStar/BookAStar/Pages/EditBillingLinePage.xaml
index 84937a09..bb281a95 100644
--- a/BookAStar/BookAStar/Pages/EditBillingLinePage.xaml
+++ b/BookAStar/BookAStar/Pages/EditBillingLinePage.xaml
@@ -3,6 +3,8 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
              xmlns:local="clr-namespace:BookAStar;assembly=BookAStar"
              xmlns:views="clr-namespace:BookAStar.Views;assembly=BookAStar"
+             xmlns:behaviors="clr-namespace:BookAStar.Behaviors;assembly=BookAStar"
+             xmlns:extensions="clr-namespace:BookAStar.Extensions;assembly=BookAStar"
              x:Class="BookAStar.Pages.EditBillingLinePage"
              Style="{StaticResource PageStyle}">
   <ContentPage.Resources>
@@ -13,6 +15,23 @@
       <Style TargetType="Button">
         <Setter Property="Style" Value="{StaticResource ButtonStyle}" />
       </Style>
+      <local:BooleanToObjectConverter x:Key="boolToStyleImage" x:TypeArguments="Style">
+        
+        <local:BooleanToObjectConverter.FalseObject>
+          <Style TargetType="Image">
+            <Setter Property="HeightRequest" Value="20" />
+            <Setter Property="Source" Value="{extensions:ImageResource Samples.Images.error.png}" />
+          </Style>
+        </local:BooleanToObjectConverter.FalseObject>
+
+        <local:BooleanToObjectConverter.TrueObject>
+          <Style TargetType="Image">
+            <Setter Property="HeightRequest" Value="20" />
+            <Setter Property="Source" Value="{extensions:ImageResource Samples.Images.success.png}" />
+          </Style>
+        </local:BooleanToObjectConverter.TrueObject>
+      </local:BooleanToObjectConverter>
+      
     </ResourceDictionary>
   </ContentPage.Resources>
   <StackLayout x:Name="mainStackLayout">
@@ -22,7 +41,8 @@
            Style="{StaticResource InputLabelStyle}"></Label>
     <StackLayout Orientation="Horizontal">
       <Entry Placeholder="Durée" Keyboard="Numeric" Style="{StaticResource BigEntry}" />
-      <Picker x:Name="picker" SelectedIndex="{Binding UnitaryCost, Mode=TwoWay}" Style="{StaticResource PickerStyle}" Title="Unité de temps">
+      <Picker x:Name="picker" SelectedIndex="{Binding UnitaryCost, Mode=TwoWay}"
+              Style="{StaticResource PickerStyle}" Title="Unité de temps">
         <Picker.Items>
           <x:String>Jours</x:String>
           <x:String>Heures</x:String>
@@ -31,12 +51,24 @@
       </Picker>
     </StackLayout>
     <Label Text="Quantité facturée" Style="{StaticResource InputLabelStyle}"></Label>
-    <Entry Text="{Binding Count, Mode=TwoWay}" Placeholder="Quantité" Keyboard="Numeric"  Style="{StaticResource BigEntry}"/>
+    <Entry Text="{Binding Count, Mode=TwoWay}" Placeholder="Quantité" Keyboard="Numeric" 
+           Style="{StaticResource BigEntry}"/>
     <Label Text="Prix unitaire" Style="{StaticResource InputLabelStyle}"></Label>
     <StackLayout Orientation="Horizontal">
-      <Entry Text="{Binding UnitaryCostText, Mode=TwoWay}" Placeholder="Prix" Keyboard="Numeric" Style="{StaticResource BigEntry}"></Entry>
+      
+      <Entry Text="{Binding UnitaryCostText, Mode=TwoWay}" Placeholder="Prix" 
+             Keyboard="Numeric" Style="{StaticResource BigEntry}" >
+        <Entry.Behaviors>
+          <behaviors:DecimalValidatorBehavior x:Name="unitCostValidator" />
+        </Entry.Behaviors>
+        <Image  x:Name="unitaryCostSuccessErrorImage"
+                  Style="{Binding Source={x:Reference unitCostValidator}, 
+                                Path=IsValid, 
+                                Converter={StaticResource boolToStyleImage}}" />
+      </Entry>
       <Label Text="€" Style="{StaticResource BigLabel}" HorizontalTextAlignment="Start"/>
     </StackLayout>
-    <Button Text="Valider cette ligne de facture" Command="{Binding ValidateCommand}" Clicked="OnValidateClicked"></Button>
+    <Button Text="Valider cette ligne de facture" Command="{Binding ValidateCommand}" 
+            Clicked="OnValidateClicked"></Button>
   </StackLayout>
 </ContentPage>
\ No newline at end of file
diff --git a/BookAStar/BookAStar/ViewModels/BillingLineViewModel.cs b/BookAStar/BookAStar/ViewModels/BillingLineViewModel.cs
index 77f816b4..e6c59fa1 100644
--- a/BookAStar/BookAStar/ViewModels/BillingLineViewModel.cs
+++ b/BookAStar/BookAStar/ViewModels/BillingLineViewModel.cs
@@ -83,25 +83,30 @@ namespace BookAStar.ViewModels
 
         protected decimal unitaryCost;
         public static readonly string unitCostFormat = "0,.00";
+        string unitaryCostText;
         public string UnitaryCostText
         {
             get
             {
-                return unitaryCost.ToString(unitCostFormat, CultureInfo.InvariantCulture);
+                return unitaryCostText;
             }
 
             set
             {
-                decimal newValue;
-                if (decimal.TryParse(value, NumberStyles.Currency,
-                    CultureInfo.InvariantCulture,
-                    out newValue))
+                if (unitaryCostText != value)
                 {
-                    SetProperty<decimal>(ref unitaryCost, newValue, "UnitaryCostText");
-                    SetProperty<bool>(ref invalidCost, false, "InvalidCost");
+                    try
+                    {
+                        data.UnitaryCost = decimal.Parse(value, CultureInfo.InvariantCulture);
+                    }
+                    catch (Exception)
+                    {
+                        // TODO Error model
+                        // UI should shoud entry as wearing a wrong value
+                        // thanks to its `Behaviors`
+                    }
                 }
-                else
-                    SetProperty<bool>(ref invalidCost, true, "InvalidCost");
+                SetProperty<string>(ref unitaryCostText, value, "UnitaryCostText");
             }
         }
         bool invalidCost;
@@ -149,18 +154,6 @@ namespace BookAStar.ViewModels
                 DurationUnit = DurationUnits.Minutes;
             }
         }
-
-        public decimal UnitaryCost
-        {
-            get
-            {
-                return decimal.Parse(this.UnitaryCostText,CultureInfo.InvariantCulture);
-            }
-
-            set
-            {
-                UnitaryCostText = value.ToString(unitCostFormat, CultureInfo.InvariantCulture);
-            }
-        }
+        
     }
 }